Broadband data and video services, on which our society and economy have grown to depend, have heretofore generally not been readily available to users on board mobile platforms such as aircraft, ships, trains, automobiles, etc. While the technology exists to deliver such services to all forms of mobile platforms, past solutions have been generally quite expensive, low data rate and/or available to only very limited markets of government/military users and some high-end maritime markets (i.e., cruise ships).
Co-pending and commonly assigned U.S. patent application Ser. No. 09/639,912 entitled “Method and Apparatus for Providing Bi-Directional Data Services and Live Television Programming to Mobile Platforms”, the disclosure of which is hereby incorporated by reference as if fully set forth herein, discloses one system for providing live television programming and bi-directional data communication to users onboard mobile platforms via one or more satellite links.
Network control of aggregate emissions is a key feature of the system as it permits the system to protect co-frequency fixed satellite service (FSS) systems from interference. More specifically, the system is responsible for managing the aggregate effective isotropic radiated power (EIRP) emissions of the airborne terminals that share a FSS transponder so as to maintain the aggregate EIRP emissions within a predetermined envelope of technical and operating parameters for a given FSS satellite to thereby meet the licensing requirements of the regulatory agencies (e.g., the FCC and the ITU) that oversee communication via FSS satellite stations. Network control of aggregate emissions is typically accomplished by controlling entry into the system and changes in the aircraft data rates.
In accurately managing the aggregate EIRP emissions, the off-axis EIRP pattern for each mobile platform, as well as the aggregate transmission of all mobile platforms, must be accurately modeled. The accuracy of the model is limited by errors that include, for example, mobile platform pointing errors, power control errors and the accuracy of the mobile platform antenna models. As discussed hereinafter, the exemplary mobile platform that will be used for purposes of discussion will be an aircraft. Those skilled in the art will understand, however, that the mobile platform may also be a ship, train, automobile or any type of vehicle.
The most direct method for accounting for errors in the off-axis EIRP density calculation is to hold a fixed margin for each error that provides a high probability of attainment (e.g., 99.7%). For example, a pointing error margin is established by determining the maximum change in off-axis EIRP density that results from a 99.7% probability error. As the change in the off-axis EIRP density is sensitive to the position and attitude (e.g., heading, pitch, roll) of the aircraft, the pointing error margin is computed for the worst-case location and attitude. Margins for every other error source in the EIRP density calculation are computed in a similar fashion that is based on a worst-case scenario, and then summed to provide a total margin. The off-axis EIRP density without errors is next computed for each transmitter, and the contributions of each transmitted are summed for the transponder along with the total margin. The resultant sum is then compared to a predetermined limit.
This method has several drawbacks that largely stem from the use of a worst-case scenario approach. Margins that are based on a worst-case location and attitude tend to be larger than necessary during the times at which the aircraft is not operating at the worst-case location and attitude, resulting in an artificially high error calculation that effectively decreases transponder capacity.
Summing the individual errors that were calculated on the basis of a worst-case scenario tends to produce a total error for each mobile platform that is larger than necessary, since it is unlikely that all of the potential errors would be in their worst-case condition simultaneously. Further, when more than one mobile platform shares a transponder, it is unlikely that errors for each of the multiple mobile platforms that are using the transponder would simultaneously be operating at their extreme values. Accordingly, when the conservative fixed error rates that are based on worst-case scenarios are repetitively overlaid onto the system (with each overlayment representing one of the mobile platforms), the error rate of the system is tremendously inflated, resulting in significant decreases to transponder capacity.
Another drawback with the use of fixed margins is that they tend to negate the impact of technological improvements to the system, which reduce errors. For example, the fixed margins are typically computed based on a specific type of terminal. Assuming that this terminal has a relatively high set of error characteristics, any terminal operating in the system having a relatively lower set of error characteristics is in effect penalized since the system cannot account for the fraction of the population of terminals that has better error characteristics. Again, this results in an artificially high error calculation that effectively decreases transponder capacity.
Accordingly, there remains a need in the art for an improved method of controlling the aggregate emissions of a system for providing bi-directional data services to mobile platforms, wherein the system employs an improved method for calculating the aggregate off-axis EIRP density envelope for the system, and wherein an improved method is employed for accounting for errors in the off-axis EIRP density calculation.